Reference is made to U.S. Pat. Nos. 3,931,729 and 6,098,447, both of which describe the use of magnetostriction and the Villari effect in measuring the pile resistance while it is being driven, and both of which are incorporated herein by reference. The Villari effect is a phenomenon that when one applies an external stress to a magnetostrictive material, such as iron, a corresponding strain will develop in the material, which strain will in turn induce a magnetic field in the material. The above patents, as well as the present invention, take advantage of this effect by using a force measuring inductive sensor to sense a force applied to a pile by a pile driver. The sensor generates a signal in response to the magnetic field caused by this force, which signal is proportional to the force on the pile.
In both patents a coil of wire was depicted as the sensor. In practice it has been found that this coil was objectionable because it was difficult to thread the coil under the bottom end of the pile or over the top of the pile.
The present invention overcomes this problem. Instead of a coil of wire, it has been discovered that an inductive belt sensor can be wrapped around the pile anytime during the driving of the pile. The signal from the belt sensor in response to stress in the pile from the pile driver is fed to a computer in the same manner as for the coil. The belt consists of a metallic strip of aluminum or copper or other conductive metallic material covered on both sides and ends by insulation material. At one end of the metallic strip an electric cable is connected which leads through a connector to a computer. The metallic strip picks up the magnetostriction signal which is proportional to the resistance the pile encounters during driving. The signal curves displayed on the computer are identical to those picked up by the wire wound coil.
U.S. Pat. No. 4,791,372 to Kirk et al discloses the use of a sensor for sensing the spin echo signals in a magnetic resonance imaging system. The sensor is disclosed as a metal sheet covered with insulation, with the ends on one side of the sheet bearing VELCRO or other hook and loop type fastener means fixed thereto. The sensor is wrapped about a patient in conforming fashion to form a coil with the facing ends of the sensor, bearing VELCRO or other hook and loop type fastener means, overlapping so that the overlapping metal sheet and intervening insulation form a capacitance in parallel with the coil. The coil/capacitance is connected to a capacitive tuning circuit to tune the coil/capacitor toward the resonance of the particular Lamor frequency being sensed.
The belt sensor of the present invention differs from that shown by Kirk et al in that the ends of the metallic foil do not overlap to form a parallel capacitor. The metallic foil of the invention also does not completely encircle the test object as does Kirk et al or conform to the test object as does Kirk et al but is spaced therefrom. Kirk et al's sensor also requires a tuning circuit, since its coil/capacitance must be tuned to resonance. The sensor of the invention is an inductive sensor only and requires no tuning.
Through tests of the foil belt sensor in the field, it has been determined that fifty tons of pile bearing equals one volt on the impact curve developed by the sensor of this invention.
Another method to calibrate the foil transducer is through the use of the dead load test for piles. By this system a pile is driven into the ground with the foil transducer wrapped around the pile in the prescribed manner. The peak voltage is noted on the computer. Then a dead load is applied to the pile until it fails to support the load. This is known as the Ultimate Bearing Capacity. The ultimate bearing capacity will be the peak voltage on the computer. By dividing the total load in tons by the voltage, one can arrive at the number of tons per volt sensed.